I posted on your blog about a problem I had compiling async with boost 1.35 and up. At the time I just kept using the old version of boost, but upgrading Ubuntu made me look at this again. I think your async library is the best thread helping library I've found by far as it is simple and easy to use, so thanks for writing it.
The problems I was encountering were minor so I figured out the change that seems to work at created a diff. I'm now using boost 1.38.
Diff for async-0.2.3/async/include/async/detail/result_store.hpp:
84c85 < if (l.locked())
145c146 < if (l.locked())
> if (l.owns_lock())